The present invention relates to a semiconductor device, such as a thyristor or a triac, including a protecting MOS transistor.
A zero-cross photo thyristor or a zero-cross photo triac, which is turned on only at a zero-cross time point at which an AC voltage applied between a cathode and an anode crosses a zero line or at a time point near the zero-cross time point, has been proposed. The zero-cross photo thyristor is not turned on upon incidence of light when the voltage applied between the anode and cathode is high, but is turned on upon incidence of light when the voltage is low and is near the zero-cross time point. In such a thyristor, noise generated during switching is significantly reduced. An example of a thyristor whose operation is controlled by an incorporated MOS transistor is disclosed in Japanese Patent Disclosure (Kokai) No. 58-105572. A semiconductor device in which a gate portion of the above MOS transistor is protected from a high voltage so as to allow the above thyristor or the triac to have a high withstand voltage is disclosed in Japanese Patent Disclosure No. 60-74678. FIG. 1 is a sectional view of a planar type thyristor having such a structure for protecting a gate portion.
The thyristor shown in FIG. 1 has a pnpn four-layered structure including n-type cathode region 1, n-type region 2, p-type base regions 3 and 4 (which are formed integrally or connected electrically) surrounding cathode region 1 and n-type region 2, n-type region 5 surrounding base regions 3 and 4, and p-type anode region 6 surrounding n-type region 5. N-type region 2 is short-circuited to base region 3 through wiring 7. Cathode region 1, n-type region 2, gate insulating layer 8 provided on cathode region 1, n-type region 2 and base region 3 between cathode region 1 and n-type region 2, and gate electrode 9 provided on gate insulating layer 8 constitute a MOS transistor. Cathode region 1 and n-type region 2 are electrically connected and disconnected according to an ON-OFF operation of the MOS transistor. P-type floating region 10 is formed in the inner region of n-type region 5 surrounded by base regions 3 and 4, and gate electrode 9 is connected to floating region 10.
An operation of the semiconductor device in FIG. 1 is as follows. Assume that a sine wave voltage is applied between anode electrode A and cathode electrode K, the anode is positive and the cathode is negative, and a voltage amplitude is high. In this state, the voltage between the anode and cathode is applied to a p-n junction between n-type region 5 and base region 3. When the voltage is applied in the manner described above and light becomes incident on a surface of the device in FIG. 1, a photo current is generated in base region 3. The thyristor is turned on by the photo current if not having a structure to be turned on near the zero-cross time point of the AC voltage. However, in the thyristor shown in FIG. 1, a channel is formed in a surface region of base region 3 between cathode region 1 and n-type region 2 according to the gate voltage. More specifically, a current path including wiring 7, n-type region 2, the above channel, cathode region 1, and cathode electrode K is formed from base region 3, and the photo current generated in base region 3 flows to cathode K. Therefore, the thyristor is prevented from being turned on when the AC voltage is high.
The thyristor is turned on when the voltage between the anode and cathode is low enough to be near the zero-cross time point. More specifically, a potential of anode electrode A is supplied to gate electrode 9 through floating region 10, and a potential of cathode electrode K is supplied to the channel. Accordingly, when the voltage between the anode and cathode is lower than threshold voltage Vth of the MOS transistor, the channel which electrically connects cathode regions 1 and n-type region 2 is not formed. When the channel is not formed as described above, the thyristor is turned on by the photo current generated in base region 3. Once the thyristor is turned on, the MOS transistor is kept in an off state since the voltage between the anode and cathode does not rise over the forward voltage drop of the thyristor.
The semiconductor device (including a thyristor or a triac) shown in FIG. 1 has the following problems. More specifically, when a thyristor, a triac, or the like has a high withstand voltage, a high voltage is applied between the anode and cathode. Therefore, the MOS transistor must be prevented from breakdown by the high voltage. For this purpose, a structure which limits the voltage applied to the gate electrode below a predetermined value has been proposed. More specifically, a p-type floating region 10 surrounded by base regions 3 and 4 is provided such that a depletion layer which is generated between p-type base regions 3 and 4 and n-type region 5 gradually extends as the voltage between the anode and cathode becomes high and at last reaches floating region 10, thereby limiting the voltage applied to gate electrode 9 at a predetermined value. The voltage between the anode and cathode when the depletion layer reaches floating region 10 is called a punch-through starting voltage.
In order to control the above punch-through starting voltage precisely, the following three factors are present. That is, (1) impurity concentration of n-type region 5, (2) distances l between base regions 3 and 4 and floating region 10, and (3) surface impurity concentrations of base regions 3 and 4 and floating region 10. Since factor (1) is normally determined by the other conditions, the controls of factors (2) and (3) are important. Factors (2) and (3) are controlled by the diffusion length in the lateral direction of base regions 3 and 4 and floating region 10.
As for a thyristor or a triac with a high withstand voltage, a diffusion depth of base regions 3 and 4 must be about 40 .mu.m. The diffusion depth, i.e., the diffusion length in the longitudinal direction and impurity can be controlled with relatively high precision. However, it is very difficult to control precisely the diffusion length in the lateral direction, so that the diffusion length in the lateral direction varies largely according to a manufacturing process. More specifically, in the structure of FIG. 1, distances l between base regions 3 and 4 and floating region 10 vary largely; so does the punch-through starting voltage. Therefore, control of distances l is important.